A car navigation system provides transportation guide information to a user by working map data. For a storage medium for map data, many models used a hard disk drive (HDD), however, recently, models using a semiconductor memory such as an SD card and a solid state drive (SSD) are being mainstream.
The semiconductor memory has advantages such as it hardly consumes power and it is excellent in shock resistance, compared with prior HDD, while the semiconductor memory has a defect that a unit price per capacity is high. Therefore, to provide a car navigation system to a user at the similar price to the prior price, the used capacity of the semiconductor memory is required to be reduced. In the meantime, as the contents of data required to operate a car navigation system are unchanged even if HDD or a semiconductor memory is used for a storage medium and besides, the size of a database has a tendency to increase year by year because of a new road and new facilities, technique for reducing the size of data recorded in the car navigation system is continuously desired. For this technique for reducing the size of data, the following techniques are proposed.
First, for a method generally used to reduce data size without changing the contents of data, data compression technique disclosed in Patent Literature 1 can be given. As for data compression techniques, multiple compression methods are proposed in addition to the technique disclosed in the patent literature 1 because data size can be easily reduced and the compression methods are widely actually used.
Besides, for another technique for reducing data size, technique disclosed in Patent Literature 2 can be given. According to this technique, when data has two-dimensional tabular structure, a dictionary is made according to a predetermined procedure based upon each column of a table, the data is compressed in units of column by utilizing the dictionary, and data size is reduced. When the data to be compressed has the two-dimensional structure configured by a row and a column as described above, it is generally known that in the compression of data in units of column, the enhancement of compressibility can be expected more than the simple processing of data (a byte string) in a direction of a row from the head and it is referred in multiple documents such as Non Patent Literature 1.
Moreover, for further another technique for reducing data size, technique disclosed in Patent Literature 3 can be given. According to this technique, from a viewpoint that random access to a specific position in a database and sequential access in referring to relatively voluminous data mix as a mode in which an application of a car navigation system accesses the database, performance in sequential access can be also enhanced by reading the following block in a memory beforehand in referring to a certain block, enabling random access to desired data after data to be compressed is divided in units of predetermined block and is compressed.